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Surfactants, oils can be overused

Bob Scott | Jun 20, 2003

At the end of almost every herbicide recommendation that I or anyone else makes, usually comes the comment “and add a quart of oil” or “don't forget a surfactant.” For most herbicide recommendations these statements are usually correct.

With some herbicides a surfactant is so important that many formulations come with one built in. The most obvious example of this is Roundup WeatherMax or other glyphosate formulations that typically say “plus” or something like that at the end.

Another example is MSMA for your yard or use in cotton. With these herbicides, the manufacturer has made the decision for you and added a surfactant to the jug.

Spray adjuvants, for the most part, are needed only when herbicides are being applied after the weeds are emerged. I am always shocked when I hear that a crop oil or surfactant was used with a pre-emerge herbicide application. The only time this should happen is if some weeds are already emerged and you hope that your pre-emerge treatment will pick up some of them.

I do not claim to be a true expert on spray adjuvants, and I know that I am stepping on some toes here (so please do not call), but from my experience the main thing about spray adjuvants is that weed control is better with one than without one.

In research trials it is very difficult to tell one adjuvant from another, especially in the more humid environment that we have here in the South. In more northern climates, a spray grade fertilizer is often used to aid in herbicide uptake. This is typically not needed in more hot and humid climates.

You often have to evaluate many locations and environments in ordr to determine which adjuvant is best.

I do not have a brand or specific adjuvant that I feel is better or worse than another in most cases. In my research program, we use Ag-98 or an equivalent for our non-ionic surfactant and Agri-dex or Prime Oil II as our crop oil. However, I probably would not hesitate in substituting another brand.

For general weed control, I like the 80/20 blends of non-ionic and oil which are good for a wide range of conditions.

More important than the brand name of an adjuvant is using the type and rate that the label calls for. Speaking from experience I know that companies typically put a considerable amount of time and effort into testing various spray adjuvants to come up with the one that is just right for their herbicide.

For example, at my former job I remember several trials that were conducted evaluating non-ionic surfactants, silicon-based surfactants and crop oils with Newpath herbicide. In the end, a simple non-ionic surfactant at 0.25 percent volume per volume was all that was needed for optimum weed control and good crop tolerance.

For some herbicides a surfactant or oil is not required. An example of this is Hoelon. Although I am aware that many use and recommend crop oils with Hoelon for ryegrass control in wheat, it is simply not needed.

With other herbicides, like propanil, the use of a spray adjuvant may cause unnecessary crop response and is, therefore, not recommended under most conditions.

Other herbicide labels get very specific. Valent Company feels very strongly, for example, that a silicon-based surfactant is needed with Regiment. The Regiment label specifically requires that Kinetic, Regiwet or an equivalent is used at 16 ounces per 100 gallons of spray or 0.125 percent volume per volume.

With some herbicides and herbicide uses, a crop oil or other adjuvant is a must. Clincher, Select, Poast and other grass herbicides require the use of a spray additive. Most of these will not work well without one.

Before Permit came along, Basagran was used more to control nutsedge in rice. Nutsedge control with Basagran is as different as night and day, with and without oil. Permit also needs a surfactant or oil, but the difference in control with and without one is not as dramatic.

One final note is on spray additive rate. I get very frustrated when I hear folks talk about crop oil or other additive rates in pints or quarts per acre. Spray additives have no herbicidal activity to speak of. They are put in the tank simply to aid in the uptake of the herbicide into to the plant. They can do this in several ways. Typically rate is not that important.

Most weed scientists and others talk about adjuvant rates in terms of percent of spray volume. For example, if you ask me what adjuvant to use with Permit herbicide on big nutsedge, I would probably tell you 1 percent crop oil concentrate. So in a 100-gallon tank, you would pour 1 gallon of crop oil, regardless of the gallons per acre being applied.

What I am hearing more and more of out there is folks giving a recommendation of 1 quart per acre. At a 5 gallon per acre spray volume that would be 20 quarts or 5 gallons of crop oil in the same 100 gallon tank! I do recommend quart per acre rates under some conditions, such as big grass control with Clincher or Facet, but these are exceptions and not the rule.

Producers should be aware of label recommendations on adjuvant use. Typically when they are overused, the only cost is in the cost of the extra product. We try to be as specific as possible with adjuvant recommendations. The herbicide label is ultimately the best source for adjuvant use recommendations.

Bob Scott is the University of Arkansas Extension weed specialist. bscott@uaex.edu.